A patient with severe abdominal pain had a stool culture revealing non-lactose-fermenting, Gram-negative rods that produce gas in nutrient broth. What is the most likely causative agent?

Correct Answer: D

Rationale: The correct answer is D: Proteus vulgaris. This is because Proteus species are non-lactose-fermenting, Gram-negative rods that produce gas in nutrient broth. They are known to cause severe abdominal pain due to their ability to invade the intestinal mucosa. A: Salmonella enterica is a lactose-fermenting organism and typically does not produce gas in nutrient broth. B: Shigella dysenteriae is a lactose-negative organism but does not produce gas in nutrient broth. C: Escherichia coli is a lactose-fermenting organism and does not typically produce gas in nutrient broth. In summary, the presence of non-lactose-fermenting, gas-producing Gram-negative rods points towards Proteus vulgaris as the most likely causative agent of the severe abdominal pain.

Correct Answer: B

Rationale: The correct answer is B: Corynebacterium diphtheriae. This bacterium produces a toxin that causes diphtheria. It forms a grayish membrane in the throat, leading to difficulty breathing. Bacillus anthracis (Choice A) causes anthrax, Streptococcus pneumoniae (Choice C) causes pneumonia, and Escherichia coli (Choice D) causes various infections but not diphtheria.

Correct Answer: A

Rationale: The correct answer is A: Motility. Native microscopic slides are ideal for observing motility because they allow for the observation of living microorganisms in their natural state. By observing the movement of microorganisms on a native slide, one can assess their motility characteristics accurately. Choice B (Division) is incorrect because native slides may not provide the ideal conditions for observing cell division, as it may be difficult to capture the exact moment of division in a live organism. Choice C (Presence of capsules) is also incorrect because observing capsules usually requires specific staining techniques that may not be suitable for native slides. Choice D (None of the above) is incorrect as native slides are indeed beneficial for observing motility.

Correct Answer: B

Rationale: The correct answer is B (2). During glycolysis, one glucose molecule is broken down into two molecules of pyruvate. In this process, a net gain of 2 ATP molecules is produced through substrate-level phosphorylation. Specifically, ATP is generated during the conversion of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate and during the conversion of phosphoenolpyruvate to pyruvate. The other choices are incorrect because the net gain of ATP molecules in glycolysis is not 1, 3, or 4; it is 2 based on the specific enzymatic reactions involved in the pathway.

Correct Answer: C

Rationale: Rationale: 1. Pseudomonas aeruginosa is commonly found in hospital environments. 2. It is an opportunistic pathogen causing various nosocomial infections. 3. Its ability to develop resistance to antibiotics makes it a significant threat. 4. Corynebacterium diphtheriae causes diphtheria, not common nosocomial infections. 5. Shigella sonnei causes foodborne illness, not typically nosocomial infections. 6. "None of the above" is incorrect as Pseudomonas is indeed commonly associated with nosocomial infections.

Correct Answer: B

Rationale: The correct answer is B: cell wall. Lysozyme is an enzyme that targets the cell wall of bacteria by breaking down the peptidoglycan layer, leading to bacterial cell lysis. This action disrupts the structural integrity of the bacteria, ultimately causing their death. Choices A, C, and D are incorrect because lysozyme does not target the cytoplasm, nucleus, or capsule of bacteria. The primary function of lysozyme is to act on the cell wall, making option B the correct answer in this context.

Correct Answer: A

Rationale: The correct answer is A: Obligate anaerobes. These bacteria can only survive in the absence of oxygen as they lack the enzymes needed for aerobic respiration. They typically use fermentation or anaerobic respiration for energy production. Incorrect choices: B: Facultative anaerobes can switch between aerobic and anaerobic metabolism based on oxygen availability. C: Obligate aerobes require oxygen for survival and cannot survive in anoxic conditions. D: Microaerophiles require low levels of oxygen for growth and are not capable of surviving in completely anoxic conditions.

Correct Answer: A

Rationale: The correct answer is A: Giardia lamblia. Giardia lamblia is a protozoan parasite that causes foul-smelling diarrhea. The trophozoites of Giardia lamblia have two nuclei and flagella, which are characteristic features seen on stool microscopy. The presence of these specific features helps differentiate Giardia lamblia from the other options. Entamoeba histolytica (B) typically presents with one nucleus and lacks flagella. Trichomonas vaginalis (C) is a sexually transmitted parasite with a different morphology. Balantidium coli (D) is a ciliated parasite, not flagellated like Giardia lamblia.

Correct Answer: C

Rationale: Correct Answer: C: dermatophytes Rationale: 1. Dermatophytes are fungi that specifically infect the skin, hair, and nails. 2. They are the most common fungi causing such infections in humans. 3. Plasmodium and sporozoites are parasites causing malaria, not skin diseases. 4. Nematodes are a type of roundworm and do not typically cause skin, hair, or nail infections. In summary, dermatophytes are the correct answer because they are specialized fungi that commonly cause skin, hair, and nail diseases, distinguishing them from the other choices.